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(11) | EP 0 317 445 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method for fabricating a silicon carbide substrate |
| (57) Two realizations of a method are disclosed for fabricating semiconductor substrate
having an S0I β -SiC on insulator structure. In the first realization, two substrates
are prepared. The first substrate is silicon coated with SIO₂ ; the second substrate
is silicon on which β -SiC is hetero-epitaxially grown. These substrates are bonded
to each other by a heating process. During the heating process, anodic bonding may
be applied. The bonded substrate is then etched or mechanically polished off from
the side of the second substrate, to expose the β -SiC layer. In the second realization,
β -SiC is grown on a silicon substrate. The surface of the substrate is coated with
SiO₂ ; then polysilicon or poly-SiC is deposited on the surface of the β-SiC side.
The substrate is then etched or mechanically polished from the side of the silicon
substrate to expose the β -SiC. |
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for fabricating a silicon carbide substrate, especially to form a beta silicon carbide layer on an insulator.
[0002] Silicon carbide (SiC) is known as a semiconductor material which has a wide band gap in its energy level compared to that of silicon, and is suitable for fabricating devices which can be used at high temperature. In particular beta silicon carbide (β-SiC) is known to be desirable for fabricating various active devices, such as transistors or field effect transistors (FETs). Details of such devices can be found, for example, in "Experimental 3C-SiC MOSFET" by Y. Kondo et al. IEEE ED Letters Vol. EDL-7. N°. 7, July 1986.
[0003] But single crystals of β-SiC usable for fabricating active devices such as transistors or FETs are still not available by the present state-of-the-art semiconductor technology. These devices are fabricated using epitaxially grown silicon carbide layers. It is considered that a better epitaxial layer of SiC may be obtained using a SiC substrate, but since a SiC substrate large enough for fabricating various devices is not available, the growth of a SiC layer is usually accomplished using a silicon substrate. Therefore, silicon substrate is presently considered to be the only practical semiconductor substrate on which β-SiC can be epitaxially grown. Such growth of crystal is called hetero-epitaxial growth, because the crystal of the substrate and the grown crystal are different from each other.
[0004] A typical structure of a FET formed in a SiC epitaxial layer is shown in Fig. 1(a), where 101 designates a silicon substrate, and 102a is a hetero-epitaxially grownβ-SiC layer. In this example, both the silicon substrate and the SiC layer are of n-type conductivity. A source region 103 and a drain region 104 are fabricated by doping p⁺ type impurities. A gate oxide layer 105 composed of silicon dioxide (SiO₂) is formed over the channel region, and a gate electrode 106 is formed over layer 105. The surface of the device is coated with a silicon dioxide layer 107. Electrodes indicated by G, S and D contact respectively the gate, source and drain regions through contact holes formed in the SiO₂ layer 107.
[0005] In such a FET structure, it has been found that the leakage current is relatively large. This is considered to be mainly due to the fact that the SiC epitaxial layer 102 formed on the silicon substrate 101 is poor in quality, whereby the pn junction formed in such SiC layer is likely to become a leaky junction, and that the conductivity of the silicon substrate 101 becomes higher than that of the SiC layer 102 at the high temperature at which the SiC devices are expected to operate (500 - 600 °C for example). Therefore, a FET such as illustrated by Fig. 1(a) can hardly be used at high temperatures. Such defects occur in a similar way with other types of devices made from SiC material.
[0006] To avoid such defects of SiC devices, it has been suggested to insert an additional layer between the SiC layer and the Si substrate in order to suppress the leakage current through the silicon substrate. For example, in "Fabrication of Inversion-Type n-Channel MOSFET's Using Cubic-SiC on Si(100)" by K. Shibahara et al., IEEE ELECTRON DEVICE LETTERS, Vol. EDL-7, NO. 12, December 1986, or "Insulated-Gate and Junction-Gate FET's of CVD-Grown β -SiC" by K. Furukawa et al. IEEE ED Letters, Vol. EDL-8, N°.2, Feb. 1987, it has been proposed to insert a further SiC layer of another conductivity type so as to form a pn junction with the former SiC layer. This pn junction prevents current leakage.
[0007] In silicon devices, SOI (semiconductor on insulator) structures are known in which a silicon crystal is grown on an insulator such as a silicon dioxide layer formed on a silicon substrate.
[0008] If it were possible to grow a β -SiC crystal on an insulator this could be effective for preventing current leakage through the substrate. A fundamental configuration is shown in Fig. 1(b). The same reference numerals designate same parts in Figs. 1(a) and 1(b). Compared to Fig. 1(a), the proposed new device of Fig. 1(b) includes a silicon dioxide layer 108 between the silicon substrate 101 and the SiC layer 102. By inclusion of this silicon dioxide layer 108, the current leakage of the SiC FET is greatly reduced, because the current leakage through the silicon substrate 101 is suppressed.
[0009] The problem remains however as to how to realize such a SOI type device formed with SiC, that is, a device with a substrate having a structure of SiC on insulator. One proposal could consist in using a β -SiC crystal as the insulating substrate. The resistivity of undoped β -SiC is very high, but large size perfect crystals of β -SiC are still not available, the size of the substrate being an important factor for fabricating various semiconductor devices.
SUMMARY OF THE INVENTION
[0010] The object of the present invention is, therefore, to provide a method which enables the fabrication of a semiconductor substrate having a β -SiC epitaxial layer grown on an insulator, that is to say an SOI structure (the semiconductor being SiC).
[0011] Another object of the present invention is to provide a large size β -SiC substrate, on which various semiconductor devices can be fabricated.
[0012] Still another object of the present invention is to reduce leakage current of devices fabricated in a silicon carbide layer.
[0013] The present invention utilizes a SiO₂ layer as the insulation layer between the SiC and the substrates, and discloses two variant methods for fabricating the SiC-SiO₂-Si structure. The first method realizes the above structure by bonding two substrates and the second method realizes the structure by hetero-epitaxial growth and etching or polishing.
[0014] In the first method, two kinds of substrate are prepared. The first is a silicon substrate which will later become a base substrate for supporting the silicon carbide layer. The surface of this substrate is first coated with a silicon dioxide layer. The second substrate is composed of a single silicon crystal on the surface of which a silicon carbide layer is hetero-epitaxially grown, in which devices will later be fabricated. These two substrates are stacked such that the silicon carbide and silicon dioxide layers are in contact and thermally treated so as to form a bond. The bonded substrate is then polished or etched off from the back side of the second substrate, so as to remove the silicon layer of that second substrate and expose the silicon carbide layer. The exposed silicon carbide layer, originally of the second substrate is left bonded on the silicon dioxide layer supported by the first substrate. Thus the SiC on SiO₂, that is a SOI structure is realized.
[0015] In the second method, a SiC layer is hetero-epitaxially grown on a silicon substrate. This SiC layer is then coated with a SiO₂ layer. Over the SiO₂ layer, a polysilicon is deposited by chemical vapor deposition (CVD). Finally, the initial silicon substrate is removed by etching such that the SiC layer is exposed.
[0016] Various modifications of above fundamental process are possible. For example, in the first method, the surface of the second substrate may be coated by SiO₂ after the SiC layer is grown on it. Inversely, the first substrate may not be coated by SiO₂ until it is stacked onto the second substrate. The first substrate may be of any other semiconductor material, such as polycrystalline silicon (polysilicon) or β -SiC. The surface of the second substrate may be further coated with phospho-silicate glass (PSG). During the thermal treatment process for bonding the substrates, an electrostatic potential can be applied between the first and second substrates to prevent the creation of voids and to increase the adhesion strength. These and other modifications and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
Fig. 1(a) is a schematic cross sectional view of a typical FET formed in a silicon carbide substrate.
Fig. 1(b) is a schematic cross sectional view of a proposed structure of a FET improved in that the current leakage is lower than that of the FET of Fig. 1(a).
Figs. 2(a) to 2(d) are schematic cross sectional views of a substrate illustrating a fundamental process according to the present invention for fabricating a SiC-on-insulator-on-semiconductor structure, wherein:
Fig.2(a) shows a first and a second substrate prepared for carrying out the process ;
Fig 2(b) shows the step in which the two substrates are stacked ;
Fig. 2(c) illustrates the step in which the second substrate is polished or etched from the back side of the stack ; and
Fig .2(d) illustrates the final step in which the substrate has a SiC-on-SiO₂-on-semiconductor structure.
Fig. 3 illustrates how an electrostatic potential is applied to the stacked substrates.
Figs. 4(a)-4(c) illustrate some modifications of the combination of the first and second substrates used for carrying out the process illustrated in Fig. 2.
Figs. 5(a)-5(d) are schematic cross sections of a substrate illustrating steps of a modified fundamental process according to the invention, using polysilicon.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] It is known that silicon carbide has two types of crystal structure, α and β -type crystals. The β -type crystal is much more desirable for fabricating active devices such as transistors or FETs. In the following description, if not specially identified, the words silicon carbide, or SiC refer only to the β -type silicon carbide (β -SiC)
[0019] The present description discloses two ways of realizing a SiC-on-insulator structure by the process according to the invention. The first way utilizes a technology of substrate bonding and the second way utilizes hetero-epitaxial growth and etching technology. This first technology will first be described
[0020] It is known in the art that silicon substrates can be bonded to one another by insertion of a silicon dioxide layer between the two substrates and subsequent treatment at high temperature. Details of such technology are disclosed in, for example, "SILICON-ON-INSULATOR (SOI) BY BONDING AND ETCH-BACK" by J. B. Lasky et al. IEDM 85, 1985 IEEE, or "Study of Si-Wafer Directly Bonded Interface Effect on Power Device Characteristics" by H. Ohasi et al. IEDM 87, 1987 IEEE. But bonding technology applicable for SiC substrate is as yet unknown. The inventor discovered that a SiC layer can be bonded on a SiO₂ layer by proper thermal treatment. Applying this discovery it has become possible to provide a substrate having a SiC-I-Si structure.
[0021] Figs. 2(a) to 2(d) illustrate a fundamental process for fabricating a SiC-SiO₂-Si substrate, that is a SiC-on-SiO₂ layer formed on a Si substrate, by bonding two kinds of substrates. These figures schematically illustrate how the cross sections of the substrates vary at each step of the fabrication process.
[0022] First, as shown in Fig. 2(a), two kinds of substrates 1 and 4 are prepared. The first substrate 1 may typically be an n type silicon substrate of about 600 µm thick. The surface of the first substrate 1 is coated by SiO₂ layer 2 of 0.5 - l µm thick. This SiO₂ layer is formed by chemical vapor deposition (CVD) or thermal oxidation. The second substrate 4 is an n type single silicon crystal typically of about 600 µm thick. On one side of the second substrate 4 is grown an n type SiC crystal 3 typically of about 0.5 µm thick.
[0023] The growth technology of the SiC layer 3 is known in the art as a hetero-epitaxial growth. In accordance with this technology, the surface of the second substrate 4 is carbonized by heating the substrate at about 1,300 °C in a flow of propane gas (C₃H₈). The gas is then switched to a propane and silane gas (SiH₄) mixture, and the temperature is regulated at 1100 -1200 °C. By this process, a SiC single crystal is epitaxially grown on the surface of the substrate 4. Further detail can be seen in for example, "Fabrication of Inversion-Type n-Channel MOSFETs Using Cubic-SiC on Si(100)" by K. Shibahara et al. IEEE ELECTRON DEVICE LETTERS, Vol. EDL-7, N°. 12, December 1986.
[0024] The first and second substrates are stacked together as shown in Fig. 2(b), and thermally treated at 600 °C for 10 min. in an inert gas ambient such as nitrogen at 0.1 Pa pressure. This process is called first thermal treatment. Then the temperature is further raised to 1100 °C and the stack is annealed for about 30 min. This process is called second thermal treatment. By these thermal treatments, the two substrates 1 and 4 are bonded to one another.
[0025] During the thermal treatment, it is desirable to place a weight on the stacked substrates to correct any slight curvature which might have been present in the substrates, and to prevent creation of voids in the bonding surface. For this purpose, a weight made of a carbon graphite plate of about 1cm thick is effective.
[0026] To prevent the creation of voids, it is also effective to apply a technique known in the art as anodic bonding. Details of this technique can be seen in "A Field-Assisted Bonding Process for Silicon Dielectric Isolation" by R. C. Frye et al., Jour. Electrochem. Soc. Vol. 133, N°. 8 August 1986. This literature discloses a technology for bonding a silicon substrate on silicon. The inventor applied the anodic bonding technique for bonding SiC on SiO₂ coated silicon, and found that it is effective in realizing a void-free bond.
[0027] According to this technique, electrical potential is applied between the first and second substrates during the first thermal treatment. The adhesion of the substrates is independent of the polarity of the electric field. The SiC layer 3 and the Si substrate 4 may be said to act as a capacitor separated by the SiO₂ layer 2. The electrostatic force which attracts the SiC layer 3 toward the second substrate 1 becomes very strong due to the thinness of the electrode (about 1 µm). The need for the application of a weight is thus eliminated.
[0028] Fig. 3 shows how the electric voltage is applied between the frst substrate 1 and the second substrate 4. The upper electrode 10 and the lower electrode 11 are made of carbon graphite plate of about 1 cm thick. Because the attractive force increases with an increase in the voltage applied between the electrodes, the higher the applied voltage, the better the result until electric breakdown occurs. In one realization, a static voltage of 200 V was applied between the upper and lower electrodes 10 and 11. The carbon graphite electrode placed on the substrate also served as a weight to eliminate curvature of the substrates and aided in the prevention of voids between the bonded surfaces.
[0029] Further, it has been found that an application of pulse voltage is preferable to the application of static voltage. This is also a known technique in the art of bonding silicon substrates. Details can be seen for example in "SOI Substrate by Bonding Technology" (written in Japanese) by Arimoto et al., Publication 30a-B-1 from 1987 Spring Convention of Japan Society of Applied Physics, March 1987. In an embodiment, pulse voltage of 500 V, 100 ms duration was applied with a repetition period of 500 ms during the first thermal treatment. By applying such technique, it became possible to obtain a void free substrate of large dimension, four inches in diameter for example.
[0030] Turning back to Fig. 2(c), after the bonding is completed, the silicon layer 4 of the first substrate is removed by mechanical polishing or chemical etching to expose the SiC layer 3 as shown in Fig. 2(d). In this removal process, it is important to take care not to remove the SiC layer 3. However, the process of removal of the Si layer 4 is facilitated by the fact that β-SiC is harder than Si, and by the fact that SiC is not removed by the etchant for Si (mixture of hydrofluoric acid and chloric acid for example). Consequently, over-polishing or over-etching can be easily avoided.
[0031] According to the aforegoing process, large-size substrates having a SiC-SiO₂-Si structure can be obtained. The ability to manufacture large-size substrate is an important factor for the fabrication of semiconductor devices at low cost. The above fundamental process can be modified in various ways. Some of these variations will now be described.
[0032] Figs. 4(a) to 4(c) show variations in the preparation stage of the first and second substrates of Fig. 2(a). Each of Figs. 4(a), (b) and (c) corresponds to the stage (a) of Fig. 2, in which the first and second substrates are prepared before they are bonded to one another. In Figs. 4(a) to 4(c), the second substrate 4 and the SiC layer 3 are shown inverted ready to be joined to the first substrate 1. In a first modification of Fig. 4(a), compared to Fig. 2(a), the first substrate 1 is not coated with SiO₂, whereas the second substrate 4 and the SiC layer 3 is coated by a SiO₂ layer 5, 0.5 - 1 µm thick, formed by CVD. The subsequent steps to bond these substrates are similar to those already described and illustrated in Figs. 2 and 3.
[0033] In a second modification of Fig. 4(b), both the first substrate 1, and the second substrate 4 which has the SiC layer 3 are coated by 0.2 µm thick SiO₂ layers 2 or 5 respectively. In a third modification of Fig. 4(c), the first substrate 1 is coated with a SiO₂ layer 2 of about O.5 µm thickness. The second substrate 4 and the SiC layer 3 are coated with a SiO₂ layer 5 of about 0.5 µm thickness, and further coated by a phosphosilicate glass (PSG) layer 6. The PSG layer is about 1 µm thick and formed by CVD. The technology for the fabrication of the SiO2 layer 5 and the PSG layer 6 are well known in the art and do not require description here. The bonding method is similar to that illustrated in Fig. 2.
[0034] A further variation of the fudamental process illustrated in Fig. 2 is possible. It consists in the substitution of α -SiC for the silicon of the first substrate 1. As has been mentioned, undoped α -SiC has a high resistivity. Although α -SiC crystals include imperfections, fairly large crystals are available. A sliced α -SiC crystal can be used as a first substrate 1 in the fundamental process illustrated in Fig. 2. Subsequent process steps are all similar to those already described. It will be apparent for one skilled in the art that α -SiC can be used for the first substrate 1 in any of the variations shown in Fig. 3, it being a mere substitute for the original substrate 1 material. This substitution permits equally well the realization of a SiC-SiO₂-SiC structure and offers the added advantage of being able to withstand higher temperature thermal treatments.
[0035] A second realization of the invention will now be described and is illustrated in Fig. 5. A second substrate 4 is made of an n type single silicon crystal typically of about 600 µm thickness. As shown in Fig. 5(a), an n type SiC crystal layer 3 typically of 0.5 µm thickness is grown over a second substrate 4 by hetero-epitaxial technology. These steps are similar to those described with respect to Fig. 2(a). The substrate is thermally oxidized at about 1,000°C to form an SiO₂ layer 5 about 0.2 µm thick over the SiC layer 3. By this thermal oxidation, the surface of the second silicon substrate 4 is also oxidized as shown in Fig. 5(b). A polysilicon layer 7 of about 500 µm thickness is then formed over the upper surface (the side of SiC layer 3) of the substrate by CVD. This CVD process is a common one in the art: dichlorosilane (SiH₂C1₂) is typically used as the reaction gas. By this CVD process, the polysilicon is primarily deposited on the upper surface (over the SiC layer 3), but the entire surface of the substrate is coated by polysilicon as shown in Fig. 5(c). The substrate is then etched or mechanically polished from the back side of the second substrate 4 to expose the SiC layer 3, as shown in Fig. 5(d), this by a process as illustrated in Fig. 2(c). In Fig. 5(d), the substrate is shown inverted as compared to its position in Fig. 5(c). In the resulting SiC-SiO₂-Si structure, the first substrate corresponding to substrate 1 of Fig. 2 is replaced by the polysilicon layer 7.
[0036] A variation of above fundamental process of the second realization is possible. As will be apparent for one skilled in the art, the polysilicon layer 7 may be replaced by polycrystalline SiC (poly-SiC). In such a variation, the poly-SiC is formed by the CVD process of Fig. 5(c). The thus fabricated SiC-SiO₂-SiC structure is capable of withstanding higher temperature thermal treatment than the Si substrate SiC, a desirable characteristic for fabrication of devices in the SiC layer; processes for fabrication of SiC devices include very high temperature processing.
1. A method for fabricating semiconductor substrate having a semiconductor on insulator
structure, said semiconductor being beta type silicon carbide (β -SiC), said method
comprising the following steps :
a) preparing a first substrate made of a first semiconductor material, and a second substrate made of silicon single crystal ;
b) forming a layer of β -SiC single crystal on one surface of said second substrate ;
c) coating the surface of said first substrate with an insulator layer ;
d) stacking said second substrate onto said first substrate with said β -SiC layer facing said first substrate ;
e) bonding said first and second substrates by heating them in an inert gas ambient ; and
f) removing said second substrate to expose said β -SiC layer.
2. A method for fabricating semiconductor substrate as set forth in claim 1, wherein
said insulator layer is a silicon dioxide layer (SiO₂).
3. A method for fabricating semiconductor substrate as set forth in claim 1, wherein
said inert gas is nitrogen.
4. A method for fabricating semiconductor substrate as set forth in claim 1, wherein
said bonding step (e) further comprises a substep of applying electric potential between
said first and second substrates.
5. A method for fabricating semiconductor substrate as set forth in claim 1, wherein
said first semiconductor material is silicon or silicon carbide.
6. A method for fabricating semiconductor substrate having a semiconductor on insulator
structure, said semiconductor being beta type silicon carbide (β -SiC) said method
comprising the following steps :
a) preparing a first substrate made of a first semiconductor material, and a second substrate made of silicon single crystal ;
b) forming a layer of β -SiC single crystal on one surface of said second substrate ;
c) coating surface of said second substrate and β-SiC layer with an insulator layer ;
d) stacking said second substrate onto said first substrate with said β -SiC layer facing said first substrate ;
e) bonding said first and second substrates by heating them in an inert gas ambient ; and
f) removing said second substrate to expose said β-SiC layer.
7. A method for fabricating semiconductor substrate as set forth in claim 6, wherein
said insulator layer is a silicon dioxide layer (SiO₂).
8. A method for fabricating semiconductor substrate as set forth in clam 6, wherein
said inert gas is nitrogen.
9. A method for fabricating semiconductor substrate as set forth in claim 6, werein
said bonding step (e) further comprises a substep of applying electric potential between
said first and second substrates.
10. A method for fabricating semiconductor substrate as set forth in claim 6, wherein
said first semiconductor material is silicon or silicon carbide.
11. A method for fabricating semiconductor substrate having a semiconductor on insulator
structure, said semiconductor being beta type silicon carbide (β -SiC), said method
comprising the following :
a) preparing a first substrate made of a first semiconductor material, and a second substrate made of silicon single crystal ;
b) forming a layer of β -SiC single crystal on one surface of said second substrate ;
c) coating said first substrate, second substrate and β-SiC layer with an insulator layer ;
d) stacking said second substrate on said first substrate with said β-SiC layer facing said first substrate ;
e) bonding said first and second substrates by heating them in an inert gas ambient ; and
f) removing said second substrate to expose said β-SiC layer.
12. A method for fabricating semiconductor substrate as set forth in claim 11, further
comprising a step of :
c') further coating said second substrate and β -SiC layer with phosphosilicate glass (PSG), said step c′) being inserted between steps c) and d).
c') further coating said second substrate and β -SiC layer with phosphosilicate glass (PSG), said step c′) being inserted between steps c) and d).
13. A method for fabricating semiconductor substrate as set forth n claim 11 or 12,
wherein said insulator is silicon dioxide (SiO₂).
14. A method for fabricating semiconductor substrate as set forth in claim 11 or 12,
wherein said inert gas is nitrogen.
15. A method for fabricating semiconductor substrate as set forth in claim 11 or 12,
wherein said bonding step e) further comprises a substep of applying electric potential
between said first and second substrates.
16. A method for fabricating semiconductor substrate as set forth in claim 11 or 12,
wherein said first semiconductor material is silicon or silicon carbide.
17. A method for fabricating semiconductor substrate having a semiconductor on insulator
structure, said semiconductor being beta type silicon carbide (β-SiC), said method
comprising the following steps :
a) forming a layer of β -SiC crystal on one surface of a silicon substrate which is made of silicon single crystal ;
b) coating the surface of said silicon substrate and β -SiC layer with an insulator layer ;
c) depositing polycrystalline silicon (polysilicon) or polycrystalline silicon carbide (poly-SiC) on the SiC side of said silicon substrate ; and
d) removing said silicon substrate to expose said β -SiC layer.
18. A method for fabricating semiconductor substrate as set forth in claims 17, wherein
said insulator layer is silicon dioxide (SiO₂).